This invention relates to a homogeneous, concentrated, prepolymer composition useful for forming polyimide articles, such as polyimide composites.
Polyimide resins are used to form structural components for military and civil aviation applications, such as jet engine cowls and ducts. Polyimides useful for these applications are lightweight, have superior load-bearing characteristics, and have glass transition temperatures (T.sub.g) substantially above the temperature at which the composite is used to preclude softening and creep. Also, the polyimide must have good thermo-oxidative stability at high temperatures (typically from 500.degree. F. to 700.degree. F.). High thermo-oxidative stability is indicated by low weight loss after long term, high temperature, exposure to oxidative environments. For example, our U.S. Pat. Nos. 5,091,505 and 5,132,395 teach polyimides which are thermally stable at temperatures of up to 700.degree. F., and our co-pending U.S. patent application Ser. No. 07/816,304, filed on Dec. 27, 1991, describes a specific formulation of monomeric reactants which form polyimides thermally stable at temperatures of up to 800.degree. F. The monomeric reactants taught by the latter application comprise: (a) an ester of biphenyltetracarboxylic acid, (b) phenylenediamine, and (c) a divalent end cap compound capable of reacting with the phenylenediamine or the ester to form an end cap radical that precludes further reaction of the phenylenediamine with the ester.
Generally, the fiber reinforced polyimide structural components comprise fibers such as glass, ceramic, or carbon fibers, embedded in a polyimide matrix. Typically, the structural component is made by the steps of (i) forming a low concentration monomeric reactant solution, (ii) impregnating fibers with the low concentration solution to coat the fibers with the monomeric reactants, (iii) forming the coated fibers into a structural component, and (iv) polymerizing the monomeric reactants on the fibers in situ to form a fiber reinforced polyimide composite. For example, our aforementioned co-pending application, Ser. No. 07/816,304, discloses a low concentration monomeric reactant solution that comprises 37% monomer by weight in methanol. Conventional solvent, such as methanol, is capable of dissolving only up to about 30% to 40% by weight of monomer. Attempting to dissolve additional monomer can cause the dissolved monomers to precipitate from the solution.
There are several problems with the use of low concentration solutions for impregnating reinforcing fibers. One problem is that the low concentration (or low solids content) of these solutions necessitates multiple fiber impregnation steps to obtain a sufficiently thick coating of the monomers onto the fibers. The multiple impregnation steps are inefficient and costly. Also, the multiple impregnation steps can result in a non-uniform coating of monomeric reactants on the fibers. The non-uniform coating results in a composite having non-uniform loading bearing properties caused by variations in distribution of fibers within the composite. Variations in fiber content can also cause separation and delamination between fiber layers.
Another problem with the low concentration monomeric reactant solutions is that a large percentage (about 50% by volume) of solvent is needed to completely dissolve the monomeric reactants. The solvent volatilizes during the curing of the composite, forming large amounts of gaseous byproducts which form voids within the molded composite. These voids further weaken the load bearing capability of the composite, and also reduce the thermo-oxidative stability of the composite.
A further problem with the use of the low concentration monomeric solutions is their relatively short shelf life. The short shelf life occurs because the monomers dissolved in the solvent precipitate out of the solution when the solution is stored for a period of time. For example, our application Ser. No. 07/816,304, discloses a methanolic reactant solution which has a shelf life of about 24 hours to about 48 hours, because when stored for a longer period of time, the ester of biphenyltetracarboxylic acid in the solution precipitates out from the solution. A short shelf life is commercially undesirable because it necessitates preparation of the monomeric reactant solutions immediately prior to their use.
Thus, there is a need for a method of preparing a substantially homogenous, concentrated, prepolymer composition suitable for uniformly coating fibers. It is also desirable for the prepolymer composition to contain reduced amounts of solvent to reduce outgassing during curing of the prepolymer and to increase the uniformity of the coating on the fibers. It is also desirable for the prepolymer composition to have an extended shelf life, be affordable and non-toxic, and be capable of use to fabricate composites using conventional molding equipment.